Acoustic imaging agent

ABSTRACT

An imaging agent, a method of production of the imaging agent, and the use of the imaging agent for microseismic monitoring of subterranean formations such as those generated during hydraulic fracturing. The acoustic emitting agent is tailorable for emission delay to ensure placement and frequency emission profiles for well region differentiation. This monitoring tool is highly useful in gas, oil, and geothermal well defining and stimulation monitoring.

The present invention is a continuation of U.S. application Ser. No.15/904,690 filed Feb. 26, 2018, which in turn claims priority on U.S.Provisional Application Ser. No. 62/469,108 filed Mar. 9, 2017, which isincorporated herein by reference.

The present invention relates to an imaging agent for microseismicmonitoring of subterranean formations such as those generated duringhydraulic fracturing. The invention pertains to an imaging agent, amethod of production of the imaging agent, and the use of the imagingagent. The invention as described herein is applicable to subterraneanformation monitoring, and is particularly useful for energy productionwells such as oil, gas, and geothermal; however, the invention is alsointended for use in other seismic monitoring applications in which is itdesirable to know flow passage.

BACKGROUND OF THE INVENTION

Hydraulic fracturing has revolutionized energy production from domesticresources, including tight oil and gas formations, and unlockinggeothermal energy. Hydraulic stimulation was first used in the 1940s,but has since evolved and is now an essential technique in thedevelopment of oil and gas reserves. These techniques are largelyresponsible for development of the Barnett Shale, Haynesville Shale,Fayetteville Shale, and Marcellus Shale gas fields. Hydraulic fracturingcan also liberate oil from tight rock units as has been done with theBakken Shale and Niobrara Shale. Estimates reported by The NationalPetroleum Council expect hydraulic fracturing to eventually account fornearly 70% of natural gas production in North America.

The production of natural gas and oil from North American sources isfeeding a manufacturing renaissance and reducing our reliance on foreignsources while facilitating a switch from coal and oil/gasoline tocleaner natural gas. The development of unconventional oil and gas(particularly gas) resources, as well as geothermal energy, remains veryexpensive and can be repeated multiple times on a well before successfulresults are realized. Reducing the cost through understanding andoptimizing completion operations in hydraulic fracturing is essentialfor continued development of unconventional energy resources.

Monitoring of microseismic/acoustic events induced by hydraulicstimulation has become a key tool in evaluation of induced fractures.Microseismic technology “listens” to the formation as it is breakingapart during a hydraulic fracture (through the use of sensors such asgeophones, fiber optic sensors and accelerometers placed in calibratedsensing arrays along with computer analysis and triangulationalgorithms) to give well operators a picture of how a well is fractured,and any faults in the formation. For more advanced applications,microseismic technology it is used to estimate the size and orientationof these induced fractures. The main goal of subterranean fracturemonitoring is to completely characterize the induced fracture structureand distribute conductivity of the fractures within a formation.Currently, the monitoring of these forming fractures has been limited tofracture stimulation while the well is actively being pumped. Thispumping and formation movement contributes to the active noise downwell. The accuracy of microseismic event mapping is dependent on thedistribution of sensitive sensors—the resulting signal to noise. Hence,signal resolution can be low and limited to microseismic mapping ofactive generated fractures, and not to the actual fractures that aremaintained by proppant. The next advancement in fracture monitoring andunderstanding is to know the location of proppant within the fractureand the distribution of fracture conductivity after pumping.

Microseismic monitoring technology, when used in combination withhydraulic fracturing, offers significant value propositions over othermethods of learning about a well site. Current advanced tools in seismicrecordings are completed with sensitive geophones or with the newerdistributed acoustic sensing (DAS). Again, their monitoring ofmicroseismic events can offer low resolution of imaged fractures andonly show active fracture formation, and not maintained fractures.

There are existing prior art patents relating to seismic generation forimproving seismic mapping of geological formations. U.S. Pat. No.3,587,775 describes a controlled explosive by using dissociative water'shydrogen and oxygen. U.S. Pat. Nos. 4,038,631; 3,909,776; 3,718,205, and3,221,833 describe the use of a mechanical impacting device. U.S. Pat.No. 4,805,726 describes a controlled charge for initiating a largehollow vessel implosion. These patents disclose the use of energy from ahydrostatic implosion from the well bore's high pressures to create aseismic source emission. However, the technology taught by the prior artstill requires wiring and layering the charge ignition, and is far toolarge to fit into fractures. Additionally, the production of machiningthe tool limits its use as a seismic source for only mapping by theinterpretation of seismic changes as the signal traverses through theformation to the sensors.

The rupture and implosive collapse of hollow cavities has been utilizedin marine studies. Examples of these works include: Orr, M. andSchoenberg, M.'s “Acoustic Signatures From Deepwater Implosions ofSpherical Cavities”, Journal of Acoustical Society of America, Vol 59,No. 5, pg. 1155 (1976); Reader, W. T. and Chertock, G.'s “TransientSounds Due to Implosions of Simple Structures Under HydrostaticPressure”, presented at 82^(nd) meeting of Acoustic Society of America(1976); And Urick, R. J. “Implosion as Sources of Underwater Sounds”,Journal of Acoustical Society of America, pg. 2026 (1964). Thedisclosures in the prior art, however, only implode at the firstexperience of the hollow cavities collapse strength limit underhydrostatic pressure, and are not designed to be able to provide thecontrolled implosive release of acoustic signal needed in subterraneanseismic (microseismic) well monitoring.

SUMMARY OF THE INVENTION

The present invention relates to an imaging agent for microseismicmonitoring of subterranean formations, and in particular to formationsgenerated during hydraulic fracturing. The invention as described hereinis applicable to subterranean formation monitoring, and is particularlyuseful for energy production wells, such as oil, gas, and geothermal.

In accordance with the present invention, the microseismic monitoring ofa formation can be improved by including an acoustic imaging agentduring the fracture packing phase and, after hydraulic fracturing, theacoustic agent will emit signals from within successful packed fractureformations. Mapping the signal of proppant placement and maintainedfracture conductivity after fracturing pressures are removed provides aninnovative step in understanding well production success and fracturingtechniques.

The present invention utilizes the native potential energy of particlesthat fracture or crush under high hydrostatic pressures deep undergroundto create a detectable acoustic sound or emission.

The present invention discloses a method for using protected particlesas an acoustic imaging agent for fracture mapping in well monitoring.There are three principle value propositions that the present inventionoffers for geologic energy recovery, namely:

1. Creating a “true” picture of a) how the well was fractures by use ofproppant, b) where proppant has gone in the well, c) if a zone wascompleted correctly; and wherein the information can be used create anactive fracture map from the formation's microseismic events during andafter hydraulic fracture pumping;

2. Enhancing the “picture” and knowledge of the well formation byincreasing the signal-to-noise ratio of microseismic event sensingduring well monitoring; and

3. Lowering the cost of microseismic monitoring though the reduction ofsensor array deployment costs and signal processing by increasing thesignal-to-noise ratio of microseismic events.

One non-limiting aspect of the present invention is the ability toenable a user to understand how and where a proppant zone is actuallylocated in the well by using proppant in combination with currentlyavailable microseismic monitoring. The sensing system of the presentinvention can be added to one or multiple zones and the frequencyresponse given by the device can be uniform or tailored for each zone sothe operator can determine how effective each fractured zone has beenformed. This information leads both to the optimization of pumpingproppant chemicals into a well and the well formation. The informationcan also be used to further increase the resolution of the fractureimage created by microseismic sensing by adding levels of detail notcurrently achievable. Furthermore, the sensing system of the presentinvention can increase the signal-to-noise ratio by increasing theamount of microseismic noises in the ground, enabling either theproduction of a clearer picture of the fracture and/or enabling the samequality imaging with less sensing equipment and processing power thatcould significantly reduce microseismic monitoring costs.

The present invention includes a powerful acoustic emission productionthat enhances microseismic well monitoring techniques to the next levelfor well management and understanding. Such acoustic emission productionallows for a clearer picture of the formation than was previouslypossible due to the enhancement of the signal-to-noise ratio and istailored to be pumped into the fractures along with proppant, giving avisual map of proppant placement, concentration, and zone fracturingeffectiveness. The invention can further enable optimizations inconsumables and water use, lower cost, decreased waste streams, and asafer hydraulic fracturing process. The invention can aid the drillingand fracturing for geothermal wells by ensuring the best and mostefficient communication path between the source and return geowell isachieved.

The invention is comprised of the engineered design and production ofacoustic imaging agents in the form of protected particles, whichprotected particles include a base particle and an outer coating thatpartially or fully coats the outer surface of the base particle, orcomposite that includes a matrix material and a plurality of baseparticles in the matrix material. The base particle can include one ormore hollow cavities; however, this is not required. The base particleis designed and configured to partially or fully fracture or collapsewhen exposed to certain external pressures, which fracturing orcollapsing of the base particle results in the generation of an acousticsound or signal. The outer coating or matrix material can be formed ofone or more materials and/or one or more layers. In one non-limitingembodiment, the outer coating or matrix material is formed of one ormore polymers and/or metals. The outer coating or matrix material istypically tailored for degradation or dissolution (collectively referredto as ‘degradation’) for the controlled delivery of the implodingacoustic signal caused or released from the fracturing or crushing ofthe base particle when exposed to certain subterranean pressures. Theprotected particles are configured to withstand initial subterraneanwell pressures; however, after a delayed period during which the outercoating partially or fully dissolves or degrades (collective referred toas ‘degrades’), the protected particle weakens and the base particle iscaused to facture or crush due to the pressures in the subterraneanenvironment, thereby creating an acoustic sound or signal that can bedetected. The use of the protected particles as localized acousticemitting agents is expected to improve signal-to-noise ratios by atleast a factor of two, thereby improving fracture imaging resolution.

One non-limiting object of the present invention is the provision ofprotected particles that can be added into a well or formation and beused to create a signal that is a readable and detectable acousticsignal or emission for use in microseismic well monitoring of the wellor other type of formation.

Another and/or alternative non-limiting object of the present inventionis the provision of a method for using protected particles that can beadded into a well or formation and be used to create a signal that is areadable and detectable acoustic signal or emission for use inmicroseismic well monitoring of the well or other type of formation.

Another and/or alternative non-limiting object of the present inventionis the provision of protected particles that are configured to at leastpartially crush or fracture to generate a readable and detectableacoustic signal or emission for use in microseismic well monitoring ofthe well or other type of formation.

Another and/or alternative non-limiting object of the present inventionis the provision of protected particles that are configured to becontrollably at least partially crushed or fractured to thereby cause areadable and detectable acoustic signal or emission.

Another and/or alternative non-limiting object of the present inventionis the provision of protected particles that are formed of a baseparticle that can optionally include one or more hollow cavities, andwhich outer surface of the base particle is partially or fully coatedwith an outer coating or matrix material, which outer coating or matrixmaterial is formulated and/or configured to inhibit or prevent the baseparticle from being crushed or fractured in a well or formation, andwhich outer coating or matrix material is configured to degrade tothereby allow the base particle to be fractured or crushed when exposedto certain pressures.

Another and/or alternative non-limiting object of the present inventionis the provision of a method of inserting protected particles into awell or formation, which protected particles are inserted into regionsof a well that would cause the base particle to be fractured or crushedbut for the outer coating or matrix material about the base particle,and wherein the material of the outer coating or matrix material overtime and/or under controlled conditions is caused to partially or fullydegrade thereby resulting in the base particle being fractured orcrushed, which fracturing or crushing results in the creation of areadable and detectable acoustic signal or emission, which acousticsignal or emission can be detected by a microseismic well monitoringsystem to be used to map fractures and/or other types of passages in awell or other type of formation.

Another and/or alternative non-limiting object of the present inventionis the provision of protected particles that are formed of a baseparticle that is coated with an outer coating or matrix material that isformed of a polymer and/or metal.

Another and/or alternative non-limiting object of the present inventionis the provision of protected particles that are formed of a baseparticle that is coated with an outer coating or matrix material, whichouter coating or matrix material is a degradable or dissolvable(collectively referred to as degradable) polymer and/or metal.

Another and/or alternative non-limiting object of the present inventionis the provision of protected particles that can be pumped with orwithout a proppant into a well.

Another and/or alternative non-limiting object of the present inventionis the provision of protected particles that create an acoustic signalor emission at a certain frequency or a certain range of frequencieswhen the base particles are fractured or crushed due to a) the protectedparticles having a particular shape, b) the base particle having aparticular shape, c) the one or more optional cavities in the baseparticle having a particular shape and/or configuration, d) pressuredifferential between the pressure in the one or more optional hollowcavities and the external pressure being exerted onto the base particle,e) material contained in the optional hollow cavity, and/or f) thematerial used to form the base particle, and wherein the createdacoustic signal or emission by the fractured or crushed base particlesis distinguishable and/or different from other sounds produced in thewell or formation, so that the generated acoustic signals or emissionsby the fractured or crushed base particles can be distinguished anddetected by a microseismic well monitoring system from other soundsgenerated in the well or formation so that the generated acoustic signalor emissions by the fractured or crushed base particles can be used tomap fractures and/or other types of passages in a well or other type offormation.

Another and/or alternative non-limiting object of the present inventionis the provision of protected particles that have a particular outercoating or matrix material composition that is formulated to controlwhen the outer coating or matrix material begins to degrade and/or therate at which the outer coating or matrix material degrades. Forinstance, the outer coating or matrix material can be formulated todegrade when exposed to certain environments (e.g., saltwater,electrolyte solutions, water, air, electromagnetic waves, sound waves,temperature, pressure, etc.).

Another and/or alternative non-limiting object of the present inventionis the provision of protected particles that have a particular outercoating or matrix material composition wherein the outer coating ormatrix material can be formulated to be inert or substantially inert tothe exposed environment until exposed to an activation condition suchas, but not limited to, temperature, electromagnetic waves, sound waves,certain chemicals, pressure and/or pH.

Another and/or alternative non-limiting object of the present inventionis the provision of protected particles wherein, once the outer coatingor matrix material has been partially or fully removed or degraded, thebase particle will fractured or crushed when exposed to high pressuresand thereby emit an acoustic signal or emission.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles to facture or crushunder hydrostatic pressure to create acoustic signals or emissions as aresult of the fracturing or crushing of the base particle of theprotected particles, which acoustic signals or emissions can be sensedby one or more subterranean sensors, such as, but not limited to,hydrophones, geophones, and fiber optics.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles that can be pumpedand placed with hydraulic fracture proppant into a well or formation,and which protected particles can be used to provide accuratemicroseismic mapping of successful fractures and their propping.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles that include a baseparticle that has a crush strength lower than the desired formationbeing mapped, and the protected particles have a crush strength that isgreater than the desired formation being mapped, so that when the outercoating or matrix material of the protected particles partially or fullydegrades, the base particle will be fractured or crushed in the desiredformation being mapped.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles that include a baseparticle that has a crush strength of 100-19,000 psi (and all values andranges therebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles that include a baseparticle that has a crush strength that is at least 1% less, andtypically about 1-1000% less (and all values and ranges therebetween)than the pressure in the desired formation being mapped.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles that have a crushstrength that is at least 1% greater, and typically about 1-1000%greater (and all values and ranges therebetween) than the pressure inthe desired formation being mapped.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles that have a crushstrength that is at least 2% greater, and typically about 2-500% greater(and all values and ranges therebetween) than the crush strength of thebase particle of the protected particles.

Another and/or alternative non-limiting object of the present inventionis the provision of the provision of the use of protected particleswherein the base particle is formed of glass, ceramic, or polymer.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the baseparticle, when fractured or crushed, emits a frequency in the range of1-10,000 Hz (and all values and ranges therebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of using protected particles wherein the base particlehas a diameter or size of 10 μm-100 mm (and all values and rangestherebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the baseparticle has an interior pressure in the one or more optional hollowcavities of 10-19,000 psi (and all values and ranges therebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the emittedfrequency when the base particle fractures or crushes is controlled bythe differential pressure between the one or more optional hollowcavities in the base particle and the pressure that is exterior to thebase particle.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the outercoating or matrix material is formulated to degrade by hydrolysis,temperature-induced softening point, temperature-induced dissolution, orby other oxidation/reduction chemistries (such as the oxidativedissolution of dissolvable magnesium alloys).

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the outercoating or matrix material is formulated from one or more polymers suchas, but not limited to, polyvinylalcohols, polycarbohydrates,polycarbonate, polylactic acid, polyglycholic acid, polyamines,polyesters, and their mixtures or copolymers thereof.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the outercoating or matrix material is formulated to sufficiently partially orfully degrade within 1-72 hours (and all values and ranges therebetween)after being inserted into the well or formation such that the baseparticle is caused to fractured or crushed within 1-100 hours (and allvalues and ranges therebetween) after the protected particles have beeninserted into the well or formation.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the volume ofthe protected particles is 0.1% to 10,000% (and all values and rangestherebetween) greater than the volume of the base particle without theouter coating.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the thicknessof the outer coating is about 1 μm to 100 mm (and all values and rangestherebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the outercoating or matrix material is applied to the base particle by one ormore process such as, but not limited to, dipping, fluidized bed spraycoating, chemical vapor deposition, suspension deposition, emulsiondeposition, extrusion, tumbling and vibratory bed spray coatingtechniques.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the outercoating or matrix material can contain one or more additives to improvethe crush resistance of the outer coating or matrix material, adjust thedegradation time of the outer coating or matrix material, and/or adjustthe density of the outer coating or matrix material.

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the outercoating or matrix material can contain one or more additives such as,but not limited to, carbon nanotubes, carbon black, nanosilica, calcium,magnesium, iron, tungsten, calcium oxide, magnesium oxide, superabsorbent polymers

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles wherein the outercoating or matrix material can contain one or more additives in theabout of 0.01-50 wt. % (and all values and ranges therebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles that can be pumpedwith proppant into a well or formation and wherein the volume percentratio of protected particles to proppant being pumped into the well orformation is 0.001-1:1 (and all values and ranges therebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of the use of protected particles that have a differentcomposition and/or configuration from the proppant.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent that can be used forsubterranean mapping of natural and induced fractures during hydraulicfracturing, wherein the acoustic imaging agent comprises a base particleand a degradable outer coating or matrix material, and optionallyreinforcing, degrading, and/or dense particle additives.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the base particleis the generating source for acoustic emission from its implosion underhydrostatic pressure, and the base particle's acoustic emission isdependent on its size, shape, shell material, shell thickness,hydrostatic crush strength, internal cavity pressure, and the externalhydrostatic crush pressure.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the base particlesize ranges from about 10 μm and 100 mm (and all values and rangestherebetween), and typically 30 μm-5 mm.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the base particleshape can be any three-dimensional shape that creates a hollow cavitysuch as, but not limited to spheroids, cubes, ovoids, irregular, orasymmetrical shapes.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the base particleis comprised of one or more materials selected from the group consistingof glass (e.g., borosilicate, soda-lime silicate, phosphates,aluminosilicate, fused silica, lead-alkali, water soluble glass, andmixtures thereof); ceramic (e.g., aluminum oxide, silicon carbide,tungsten carbide, silicon nitride, titanium oxide, titanium carbide,barium titanate, boron nitrate, water soluble ceramics, and theirmixtures thereof); or polymer (e.g., epoxies, phenolics, polyurethanes,polyamides, polyamines, polyaryls, polyethers, polyesters, polyvinyls,polyalkanes, polycarbonates, and their mixtures thereof).

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the thickness ofthe wall of the base particle is about 0.1 μm-200 mm (and all values andranges therebetween), and typically 1-50 μm.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the crush strengthof the acoustic imaging agent is about 1,000-20,000 psi (and all valuesand ranges therebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the resistance tocrush can be higher than the hydrostatic well stress pressures rangingfrom about 1,000-20,000 psi (and all values and ranges therebetween),but then becomes susceptible to the well stress hydrostatic crush as theouter coating or matrix material degrades.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein a frequencybandwidth of the acoustic signal or emission created by the fracturingor crushing of the base particle is about 0.4-10,000 Hz (and all valuesand ranges therebetween), and typically from about 0.-1,000 Hz atamplitude above 1 dB (e.g., 10⁻⁶).

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein an internal cavitypressure in the base particle can be from about 10-19,000 psi (and allvalues and ranges therebetween) to change the differential pressurebetween the cavity interior of the base particle and the externalhydrostatic well pressure, and the selection of the pressure in thecavity can be used to control the frequency of the acoustic signal oremission created by the fracturing or crushing of the base particle.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein a degradable outercoating or material reinforces the base particle against wellhydrostatic pressures, and the outer coating or matrix material can beformulated to degrade after about 30 minutes to 700 hours (and allvalues and ranges therebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the degradation ofthe outer coating or matrix material is initiated by subterranean wellconditions over a period of time during the well's stimulationoperation, during, or after hydraulic fracture pumping.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the outer coatingor matrix material is formulated to degrade at well temperatures ofabout 30-200° C. (and all values and ranges therebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the outer coatingor matrix material is caused to degrade in the chemical environment ofthe well, wherein the chemical environment can include brine solutionsin the range of 0-500,000 ppm (and all values and ranges therebetween),alkaline or acidic pH in the range of 2-14, other chemical reactionsinvolving oxidation or reduction, or combinations thereof.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the period of timeuntil the base particle fractures or crushes is about 1-72 hours (andall values and ranges therebetween), and typically from about 3-6 hours.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the outer coatingor matrix material is comprised of a polymer or metal, or a mixturethereof.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the polymer thatpartially or fully forms the outer coating or matrix material iscomprised of, but not limited to, polyvinylalcohols, polycarbohydrates,polycarbonates, polylactic acid, polyglycholic acid,poly(lactic-co-glycolic acid), polyamines, polyesters, and/or mixturesthereof; wherein the metal is comprised of, but not limited to,dissolvable metals like magnesium, calcium, and their alloys.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein crush strength ofthe acoustic imaging agent is 10-20,000% (and all values and rangestherebetween), and typically from about 50-1,000% greater than the crushstrength of the base particle.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the coatingthickness of the outer coating is about 10 nm to 1,000 μm (and allvalues and ranges therebetween), and typically from about 10 nm to 100um.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the base particlesin the matrix material constitute 0.01-60 vol. % of the acoustic imagingagent (and all values and ranges therebetween), and typically from about10-40%.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the outer coatingis applied to the base particle by using spray, solution deposition,direct application, or other standard application methods, whichincludes, but is not limited to, fluidized bed spray coating, chemicalvapor deposition, suspension deposition, emulsion deposition, tumblingand vibratory bed spray coatings.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the acousticimaging agent has a diameter or averaged width of about 10 μm-10 mm (andall values and ranges therebetween), and typically from about 35 μm-1mm.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the outer coatingis applied to the base particle by melt mix, extrusion, sprayaggregation, sintering, casting or other standard compounding methods.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the dense particleadditives are added to the outer coating or matrix material to adjustthe agent's density to match the proppant that is pumped into the wellfractures.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the base particleshave a density of 1.7-22.0 g/cc (and all values and rangestherebetween).

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the additives tothe outer coating or matrix material can be powders of iron, tungsten,or mixtures thereof.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the additives tothe outer coating or matrix material are of a size in the range fromabout 10 nm-1 mm (and all values and ranges therebetween), and typicallyfrom about 100 nm-100 μm.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the additives tothe outer coating or matrix material increases the density of theacoustic imaging agent to about 0.5-5 g/cc (and all values and rangestherebetween), and typically from about 1-3 g/cc.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the additives tothe outer coating or matrix material constitute 0-10% volume (and allvalues and ranges therebetween) of the outer coating or matrix materialand include, but are not limited to, carbon black, carbon nanotubes,silica, and calcium carbonate.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the additives tothe outer coating or matrix material constitute 0-30% volume (and allvalues and ranges therebetween) of the outer coating or matrix materialand include, but are not limited to, magnesium oxide, calcium oxide,calcium, magnesium, and super absorbent polymers.

Another and/or alternative non-limiting object of the present inventionis the provision of subterranean mapping of propped natural and inducedfractures during or after hydraulic fracturing by acoustic imaging agentis done in conjunction with well monitoring sensing arrays.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent that is mixed withproppant and then pumped into natural and induced fractures.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the agent is mixedwith the proppant at a volumetric loading ratio of agent to proppant ina range from about 0.01-10% (and all values and ranges therebetween),and typically from about 0.1-2.0%.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the agent emitsits signal after the delayed time span to be sensed and recorded by thewell's sensor arrays.

Another and/or alternative non-limiting object of the present inventionis the provision of an acoustic imaging agent wherein the recordedsignal by the well's sensor arrays provides well fracture informationthat includes proppant placement (three-dimensional map of themaintained fracture network) and at a higher resolution, then comparedto mapping active fracture formations due to the agent's generation ofmicroseismic signal increasing the sensor array's recordedsignal-to-noise ratio.

These and other advantages of the present invention will become moreapparent to those skilled in the art from a review of the description ofthe preferred embodiment and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a coated hollow particle sphere acoustic imagingagent in accordance with the invention; and,

FIG. 2 illustrates a compounded hollow particle sphere acoustic imagingagent in accordance with the invention.

DETAILED DESCRIPTION OF NON-LIMITING EMBODIMENTS

The present invention relates to an imaging agent for microseismicmonitoring of subterranean formations such as those generated duringhydraulic fracturing. The invention also pertains to an imaging agent, amethod of producing the imaging agent, and the use of the imaging agentin a well or formation. The invention as described herein isparticularly applicable to the use of an imaging agent for subterraneanformation monitoring of energy production wells such as oil, gas, andgeothermal; however, it will be appreciated that the invention is alsointended for use in other seismic monitoring applications in which is itdesirable to know flow passage in a well or other type of subterraneanformation.

The present invention utilizes the implosion energy of a base particlein a protected particle as the base particle fractures or collapsesunder hydrostatic pressure for microseismic monitoring of subterraneanformations. The fracturing or collapsing of the base particle generatesan acoustic sound or emission that can be sensed by one or more sensors(e.g., subterranean sensors, etc.) such as hydrophones, geophones, andfiber optic sensors.

The protected particle is configured to be a pumpable microseismicemitting agent that can be pumped and placed with hydraulic fractureproppant in a well or formation, and which protected particle can beused to provide accurate microseismic mapping of the well or formationduring and/or after the fracturing and/or propping of the well orformation. Two non-limiting examples of protected particles inaccordance with the present invention are illustrated in FIGS. 1 and 2 .

Referring now to FIG. 1 , there is illustrated protected particle 1 thatis formed of a base particle 3 and an outer coating 4 that is coated onthe outer surface of base particle 3. Base particle 3 includes a hollowcavity 6. The thickness of the outer coating is illustrated as beinggreater than the thickness of the shell of the base particle; however,this is not required. As can be appreciated, the base particle can beabsent hollow cavity 6. As can also be appreciated, the base particlecan optionally include more than one cavity. The hollow cavity can befilled with one or more gasses; however, this is not required. Thepressure inside the hollow cavity can be controlled; however, this isnot required. In one non-limiting arrangement, the pressure in thehollow cavity can be 0-20000 psi (and all values and rangestherebetween). In one particular configuration, the pressure in thehollow cavity is about 0-1000 psi, and typically about 0-100 psi, andmore typically about 10-20 psi (e.g., 14.7 psi). When a gas is includedin the hollow cavity, the type of gas is non-limiting (e.g., air,nitrogen, oxygen, etc.). The base particle can be formed of a variety ofmaterials such as, but not limited to, glass, ceramic, or polymer. Inone non-limiting configuration, the base particle is a hollow sphereformed of a glass material such as, but not limited to, borosilicate orsoda-lime silicate spheres. In another non-limiting configuration, thebase particle is formed of degradable sodium silicate which may or maynot include a hollow cavity. The size or diameter of the base particleis generally about 10 μm-100 mm (and all values and rangestherebetween), and typically from about 30 μm-1 mm. In one non-limitingconfiguration, the size or diameter of the base particle is about 30-990μm and the shape of the base particle is generally spherical. The crushstrength of the base particle is generally 100-19000 psi (and all valuesand ranges therebetween).

The outer coating is generally formed of a material that is formulatedto degrade by one or more mechanisms such as, but not limited to,hydrolysis, temperature softening, dissolution, and/or anoxidation/reduction reaction (such as the oxidative dissolution ofdissolvable magnesium alloys, dissolvable aluminum alloys, calcium,magnesium, dissolvable magnesium-nickel alloys, or other dissolvablemetals or dissolvable metal alloys). Non-limiting examples ofdissolvable metal materials that can be used are disclosed in U.S. Pat.No. 9,757,796 and US 2015/0299838, which are incorporated herein byreference. In one non-limiting configuration, the outer coating includesone or more polymers susceptible to hydrolysis, temperature-inducedsoftening and/or temperature-induced dissolution. Non-limiting examplesof polymers that can partially or fully form the outer coating include,but are not limited, to polyvinylalcohols, polycarbohydrates,polycarbonate, polylactic acid, polyglycholic acid, polyamines,polyesters, polyether, polyamine, polyacetal, polyvinyl, polyurethane,epoxy, polysiloxane, polycarbosilane, polysilane, and polysulfone andtheir mixtures or copolymers thereof. The thickness of the outer coatinggenerally is 2 μm-100 mm (and all values and ranges therebetween), andtypically 2 μm-60 mm. The outer coating can be applied to the baseparticle by any number of techniques such as, but not limited to,dipping, fluidized bed spray coating, chemical vapor deposition,suspension deposition, emulsion deposition, tumbling and vibratory bedspray coatings.

Referring now to FIG. 2 , there is illustrated a protected particle 2that is formed of a plurality of base particles 3 that are held togetherby a matrix material 5. Matrix material 5 can be formed of the samematerial as outer coating 4 as discussed with reference to FIG. 1 .Likewise the base particle 3 can be the same as the base particle asdiscussed with reference to FIG. 1 . The volumetric ratio of the baseparticle to the matrix material in the protected particle 0.01-100:1(and all values and ranges therebetween), and typically 0.01-0.5:1.

The size of the protected particles is generally greater than 10 μm andtypically less than about 200 mm (and all values and rangestherebetween).

The protected particle is configured to survive its delivery to thedesired location in the well or formation. The use of the outer coatingor matrix material increases the resistance of the base particle tofracturing or crushing by the formation's hydrostatic pressure. Theouter coating or matrix material is formulated to degrade over time inthe well or formation, thereby resulting in the eventual fracturing orcrushing of the base particle. As such, the outer coating or matrixmaterial is formulated to delay the fracturing or crushing of the baseparticle until the protected particle has been placed within the well orformation. Generally, the protected particle is fed into the well orformation with a proppant; however, this is not required. The fracturingor crushing of the base particle results in the creations of an acousticsignal or emission that can be detected for use in mapping the well orformation.

The base particles that are used in the protected particle are designedto have a crush strength that is less than the desired formation to bemapped. For example, if a location in a well to be mapped has a pressureof 6,000 psi, a base particle should be selected to have a crushstrength of less than 6000 psi (e.g., base particle having a crushstrength of 4,000 psi or less, etc.). Generally, the crush strength ofthe base particle is at least 100 psi less than the pressure in the wellor formation to be mapped, typically at least 500 psi less than thepressure in the well or formation to be mapped, and more typically atleast 750 psi less than the pressure in the well or formation to bemapped. If the crush strength of the base particle to too close to orgreater than the pressure in the well or formation to be mapped, thebase particle may not or will not fracture or crush, this not create theacoustic signal or emission to be used to map the well or formation.

It has been found that the frequency of the generated acoustic sound oremission caused by the fracturing or crushing of the base particle isdependent on the size of the base particle and the difference inpressure between the hollow base particle's interior pressure and theformation pressure. Generally, the frequency of the generated acousticsound or emission caused by the fracturing or crushing of the baseparticle is in the range of 1-10,000 Hz (and all values and rangestherebetween), and more typically from about 1-1,000 Hz. To generatesuch frequencies, 1) the size of the base particle is generally about 10μm to 100 mm (and all values and ranges therebetween) in size ordiameter, and typically about 30 μm to 1 mm in size or diameter, and 2)the interior pressure in the hollow cavity of the base particle is about10 psi to 19,000 psi (and all values and ranges therebetween).Controlling the microseismic emission or signal frequency by internalpressure modification in the base particle changes the differentialpressure between the hollow cavity in the base particle and the exteriorwell pressure; thus, the frequency of the emission or signal caused bythe fracturing or crushing of the base particle can be tailored. Thefollowing equation can be used to calculate the frequency of theemission or signal caused by the fracturing or crushing of the baseparticle in the well or formation.

$\begin{matrix}{\omega_{o} = {\frac{1}{R_{o}}\sqrt{\frac{3\gamma p_{o}}{\rho}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Where ω_(o) is the resonant frequency ρ is the density of the liquidR_(o) is the average size of the resonating balloon and P_(o) is theaverage pressure

By using the ideal gas law P₁V₁=P₂V₂R_(o) can be found by

$\begin{matrix}{R_{o} = \sqrt[3]{\frac{P_{g}R_{i}^{3}}{P_{l}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Where P_(g) is the pressure inside the balloon and P₁ is the pressure ofthe surrounding fluid at burst.

From these two equations, a trend for resonant frequency vs. size andpressure can be obtained.

The protected particle includes a degradable outer coating or matrixmaterial to increase the resistance of the base particle to beingfractured or crushed by the formation pressure until the protectedparticle has been delivered to a desired location in the well orformation. The outer coating or matrix material is thus formulated toprovide temporarily protection to the base particle so that it may bedelivered to the formation fractures with the proppant. The outercoating or matrix material is formulated to degrade and thus fracture orcrush the base particles, creating an acoustic signal or emission thatcan be detected and used to map the well or formation.

Common subterranean well formation conditions can range in load stresspressures of 2,000-20,000 psi, temperatures ranging from about 30-200°C., and include ionic solutions at pH from about 2-12 (the pH typicallydue to fracturing fluids). Therefore, the outer coating or matrixmaterial can be selected and coating thickness selected to 1) providecrush and/or fracture protection to the base particle as the protectedparticle is inserted into the formation so that the base particle is notfractured or crushed prior to the protected particle being pumped,inserted or otherwise positioned in the desired location in theformation, and 2) sufficiently degrade during a certain period of timeto fracture or crush the base particles and emit an acoustic signal oremission. The use of the outer coating or matrix material enables theprotected particle to be pumped into a formation and placed in a desiredlocation in the formation without premature signal emission (e.g., thebase particle is not prematurely fractured or crushed prior to beingpumped or otherwise placed in the desired location in the formation),and the outer coating or matrix material have a short enough degradationduration (e.g., less than 420 hours, etc.) to fracture or crush the baseparticle and to create an acoustic signal or emission that can bedetected during the monitoring of the formation in a time efficientperiod.

The degrading of the outer coating or matrix material can be byhydrolysis, temperature softening point/dissolution, or by otheroxidation/reduction chemistries (e.g., oxidative dissolution ofdissolvable magnesium alloys, etc.). As such, the selection of thematerial used for the outer coating or matrix material is dependent onthe formation's conditions and needs to match the temperature andchemical interaction for degradation to take place in a timely manner.For example, in a typical subterranean well formation, the conditionsencountered can be 90° C. with formation load stress pressure at 8,000psi, and the pumping solution in the formation can be 2 wt. % KCl at apH of 7. For such subterranean well formation conditions, one polymerfor the outer coating or matrix material can be poly(lactic-co-glycolicacid) (PLGA), a co-polyester susceptible to degradation by hydrolysis attemperatures of 80-90° C. over a period of 12-24 hours. In more extremesubterranean well formation conditions (e.g., higher temperatures andpressures), other polymers can be used, such as polyamides or polyarylswhich will degrade over a period of 6-24 hours.

The thickness of the outer coating or thickness of the matrix materialabout the base particle, in combination with the type of material of theouter coating or matrix material, is selected to provide the desiredfracture or crush protection to the base particle for a sufficientperiod of time to enable the protected particle to be inserted, pumpedor other positioned in the desired location in the formation. Thecoating thickness of the outer coating or composite ratio of the matrixmaterial to the base particles is dependent on 1) the type of materialused for the outer coating or matrix material, 2) the material used toform the base particle, 3) the thickness of the shell of the baseparticle when the base particle includes one or more cavities, 4) thesize of the base particle, 5) the crush strength of the base particle,6) the formation pressure where the protected particle is to be located,7) the temperature in the formation in which the protected particle isto be located, and 8) the composition and pH of the fluid in theformation in which the protected particle is to be located. Generally,the volumetric increase from the base particle to the protected particledue to the addition of the outer coating to the base particle is about0.01-10,000% (and all values and ranges therebetween). For example, inone non-limiting embodiment of the present invention, a 40 μm hollowsphere particle having an inherent crush strength of 4,000 psi includesa polymer outer coating of 60 μm to that the protected particle can beexposed to pressures of 8,000 psi without resulting in the fracturing orcrushing of the base particle. In this non-limiting embodiment, thevolumetric increase from the base particle to the protected particle dueto the coating of the polymer material on the outer surface of the baseparticle is about 1563%.

The outer coating or matrix material can optionally include additivesfor the purpose of further improving the crush resistance of theprotected particle, controlling degradation time of the outer coating ormatrix material, and/or adjusting the density of the protected particleso that it can be properly pumped into a formation. The size of theadditives are generally less than 100 μm, and typically less than 1 μm.Non-limiting additives for improving crush resistance include, but arenot limited to, carbon nanotubes, carbon black, and nanosilica. Suchadditives (when used) constitute about 0.001-10 vol. % of the outercoating or matrix material (and all values and ranges therebetween).When such reinforcing or crush strength enhancement additives are used,such use of the additives increases the crush strength of the protectedparticle by 5-20,000% (and all values ad ranges therebetween), andtypically about 10-5000%, and more typically about 10%-1000%.Non-limiting additives for controlling degradation time of the outercoating or matrix material include, but are not limited to, calcium,magnesium, calcium oxide, magnesium oxide, and super absorbent polymersor mixtures thereof. The size of the additives is generally less than1000 μm, and typically less than 100 μm. Such additives (when used)constitute about 0.001-30 vol. % of the outer coating or matrix material(and all values and ranges therebetween). When such additives to controldegradation time are used, such use of the additives typically reducesthe time of degradation of the outer coating or matrix material by5-5000%, and typically by 10-500%, and more typically by 10-100%.Non-limiting additives for adjusting the density of the outer coating ormatrix material are materials having a density of at least 1.7 g/cc,typically at least 5 g/cc, and more typically at least 6.5 g/cc.Non-limiting examples of additives that can be used to adjust densityinclude, but are not limited to, nano- and micro-powders of one or morehigh density metals (e.g., iron, copper, lead, steel, tungsten, etc.).Such additives (when used) constitute about 0.001-10 vol. % of the outercoating or matrix material (and all values and ranges therebetween). Thesize of the additives are generally less than 100 μm, and typically lessthan 1 μm. When such additives to adjust density are used, such use ofthe additives typically increase the density of the protected particleby 5-1000%, and typically by 5-100%, and more typically by 5-50%.

When the protected particle is to be pumped into a formation with aproppant, the size of the protected particle to the proppant isgenerally similar. In one non-limiting embodiment, the size ratio of theprotected particle to the proppant is generally about 0.8-1.4:1, andtypically about 0.9-1.1:1. Also, the density of the protected particleto the proppant is similar when the protected particle is to be pumpedinto a formation with a proppant. In one non-limiting embodiment, thedensity ratio of the protected particle to the proppant is generallyabout 0.8-1.4:1, and typically about 0.9-1.1:1. As such, the size anddensity of the protected particle can be selected to match or closelymatch proppant density and size so that the protected particle canreplicate the pumping and placement performance of the proppant so as tomatch placement of the proppant within the fractures of the formation.When the protected particle is to be pumped into a formation with aproppant, the protected particle is generally added to the proppantslurry such that there is more proppant in the slurry than protectedparticles. In one non-limiting embodiment, the volume ratio of theprotected particle to the proppant in the slurry that is pumped orotherwise inserted into the formation is about 0.00001-0.1:1 (and allvalues and ranges therebetween), and typically 0.0001-0.05:1, and moretypically 0.0001-0.01:1.

The following are non-limiting specific examples of protected particlesin accordance with the present invention:

EXAMPLE 1

A protected particle was formed of a base particle having an averagediameter of 30 μm. The base particle is a hollow sphere having a shellthickness of 1 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The density of the base particle is 0.38 g/ccand the crush strength of the base particle is 2,000 psi. The outersurface of the base particle was coated with a hydrolysable polyamine byspray coating deposition. Two different batches of protected particleswere formed wherein the first batch had an outer coating thickness of 10nm and the second batch has an outer coating thickness of 100 nm. Thecrush strength of both batches of protected particles exceeded 2,000psi. The two batches of protected particles were inserted into a wellformation and the fracturing or crushing of the base particle resultedin the creation of an acoustic sound or emission at a frequency of100-20,000 Hz, and the timing of the creation of the acoustic sound oremission from the two batches of protected particles was different dueto the different coating thicknesses on the two batches of protectedparticles. The protected particles having an outer coating thickness of10 nm resulted in the creation of the acoustic sound or emission about10 hours after the protected particles were pumped into the wellformation. The protected particles having an outer coating thickness of100 nm resulted in the creation of the acoustic sound or emission about400 hours after the protected particles were pumped into the wellformation.

EXAMPLE 2

A protected particle was formed of a base particle having an averagediameter of 100 μm. The base particle is a hollow sphere having a shellthickness of 3 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The density of the base particle is 0.24 g/ccand the crush strength of the base particle is 1,500 psi. The outersurface of the base particle was coated with a polyvinylalcohol coatingthat includes 5 wt. % fumed silica nanoparticles. The fumed silicananoparticles were added to increase the crush strength of the protectedparticle. The outer coating thickness was 60 nm. The resulting protectedparticle has a crush strength of over 6,000 psi. The protected particlewas configured to be pumpable into a formation with a proppant.

EXAMPLE 3

A protected particle was formed of a base particle having an averagediameter of 100 μm. The base particle is a hollow sphere having a shellthickness of 3 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The density of the base particle is 0.24 g/ccand the crush strength of the base particle is 1,500 psi. The outersurface of the base particle was coated with a polyvinylalcohol coating.The outer coating thickness was 60 nm. The resulting protected particlehas a crush strength of about 4,000 psi. The protected particle wasconfigured to be pumpable into a formation with a proppant. As isevident from Examples 2 and 3, the addition of additives to the outercoating can be used to change the crush strength of the protectedparticle.

EXAMPLE 4

A protected particle was formed of a base particle having an averagediameter of 100 μm. The base particle is a hollow sphere having a shellthickness of 3 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The density of the base particle is 0.24 g/ccand the crush strength of the base particle is 1,500 psi. The outersurface of the base particle was coated with a poly(lactic-co-glycolicacid) (PLGA) that included 5 wt. % CaO. The CaO was added to the outercoating to increase the rate of degradation of the outer coating in thewell formation. The resulting protected particle has a crush strength ofover 6,000 psi. The protected particle was configured to be pumpableinto a formation with a proppant. The protected particles in the wellformation began to create acoustic sounds or emissions due to thefracturing or crushing of the base particle about 7 hours after theprotected particles were pumped into the well formation and the creationof the acoustic sounds or emissions continued for up to 2 hoursthereafter.

EXAMPLE 5

A protected particle was formed of a base particle having an averagediameter of 100 μm. The base particle is a hollow sphere having a shellthickness of 3 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The density of the base particle is 0.24 g/ccand the crush strength of the base particle is 1,500 psi. The outersurface of the base particle was coated with a poly(lactic-co-glycolicacid) (PLGA). The resulting protected particle has a crush strength ofover 6,000 psi. The protected particle was configured to be pumpableinto a formation with a proppant. The protected particles in the wellformation began to create acoustic sounds or emissions due to thefracturing or crushing of the base particle about 12 hours after theprotected particles were pumped into the well formation and the creationof the acoustic sounds or emissions continued for up to 6 hoursthereafter. As is evident from Examples 4 and 5, the addition ofadditives to the outer coating can be used to change the degradationtime of the outer coating of the protected particle.

EXAMPLE 6

A protected particle was formed of a plurality of base particles havingan average diameter of 100 μm. The base particle is a hollow spherehaving a shell thickness of 1.4 μm. The interior pressure in the hollowcavity of the base particle is 14.7 psi. The density of the baseparticle is 0.1 g/cc. The plurality of base particles was mixed with amatrix material formed of poly(lactic-co-glycolic acid) (PLGA) and 10wt. % micron tungsten powder. The micron tungsten powder was added tothe PLGA to increase the density of the protected particle. The microntungsten powder was formed from a filament of tungsten having a 1 mmdiameter. The filament was chopped into the desired size. The baseparticles constituted 30 vol. % of the protected particle. The protectedparticle was formed through melt mixing extrusion of the base particleswith the matrix material. The protected particle had a density of about2.8 g/cc.

The protected particles were added to a proppant slurry and constitutedabout 0.1 wt. % of the proppant slurry. The proppant slurry with theprotected particles was pumped into the fracturing zones of a well. Thematrix material of the protected particles degraded by hydrolysis andthe rate of degradation only increased to an appreciable rate once theprotected particles encountered the higher temperatures within the deepwell's fractures (around 60-100° C.) at pH of 6-8. In the highertemperature environment (60-100° C.) and exposed to fluids at a pH of6-8, the matrix material degraded within 24 hours thereby releasing thebase particles from the protected particle, thus resulting in thefracturing and crushing of the base particles. The fracturing orcrushing of the base particles resulted in acoustic sounds or emissionsbeing created at a certain frequency which were detected by sensorarrays. The recorded signals from the cumulative sensor arrays were theninterpreted with modeling software to identify source locations of thesignals, and such information was then used to mapping out proppantplacement in the well, and to determine where successful fracturing hadoccurred in the well.

EXAMPLE 7

A protected particle was formed of a base particle having an averagediameter of 40 μm. The base particle is a hollow sphere having a shellthickness of 1 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The density of the base particle is 0.38 g/ccand the crush strength of the base particle is 1,500 psi. The outersurface of the base particle was coated with a PLGA by suspensiondeposition. The coating thickness was 60 μm. The protected particle hada crush strength of about 8000 psi. The protected particles weresubjected to well conditions of 30,000 ppm brine solution at a pH of7.5, a temperature of 90° C., and under 6,000 psi hydrostatic pressure.After about 12 hours and over the period of 2 hours thereafter, the baseparticles were fractured or crushed. The acoustic sound or emissioncreated by the fractured or crushed base particles had a traceableharmonic resonant frequency peak at 1,500 Hz.

EXAMPLE 8

A protected particle was formed of a base particle having an averagediameter of 20 μm. The base particle is a hollow sphere having a shellthickness of 1 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The density of the base particle is 0.38 g/ccand the crush strength of the base particle is 1,500 psi. The outersurface of the base particle was coated with a PLGA by suspensiondeposition. The coating thickness was 18 μm. The protected particle hada crush strength of about 8000 psi. The protected particles weresubjected to well conditions of 30,000 ppm brine solution at a pH of7.5, a temperature of 90° C., and under 6,000 psi hydrostatic pressure.After about 12 hours and over the period of 2 hours thereafter, the baseparticles were fractured or crushed. The acoustic sound or emissioncreated by the fractured or crushed base particles had a traceableharmonic resonant frequency peak at 3,000 Hz. As is evident fromExamples 7 and 8, the protected particle can be tailored by usingdifferent sized base particles to create a certain frequency or range offrequencies when the base particle fractures or crushes.

EXAMPLE 9

A protected particle was formed of a base particle having an averagediameter of 40 μm. The base particle is a hollow sphere having a shellthickness of 1 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The density of the base particle is 0.38g/cc. The outer surface of the base particle was coated with a PVA bysuspension deposition. The coating thickness was 68 μm. The protectedparticle had a crush strength of about 8000 psi. The protected particleswere subjected to well conditions of 10,000 ppm brine solution at a pHof 8, a temperature of 80° C., and under 6,000 psi hydrostatic pressure.After about 36 hours and over the period of 72 hours thereafter, thebase particles were fractured or crushed. The acoustic sound or emissioncreated by the fractured or crushed base particles had a traceableharmonic resonant frequency peak at 1,500 Hz.

EXAMPLE 10

A protected particle was formed of a base particle having an averagediameter of 40 μm. The base particle is a hollow sphere having a shellthickness of 1 μm. The interior pressure in the hollow cavity of thebase particle is 1000 psi. The density of the base particle is 0.39g/cc. The outer surface of the base particle was coated with a PVA bysuspension deposition. The coating thickness was 68 μm. The protectedparticle had a crush strength of about 8000 psi. The protected particleswere subjected to well conditions of 10,000 ppm brine solution at a pHof 8, a temperature of 80° C., and under 6,000 psi hydrostatic pressure.After about 36 hours and over the period of 72 hours thereafter, thebase particles were fractured or crushed. The acoustic sound or emissioncreated by the fractured or crushed base particles had a traceableharmonic resonant frequency peak at 600 Hz. As is evident from Examples9 and 10, the protected particle can be tailored by using certainpressures in the hollow cavity of the base particle to create a certainfrequency or range of frequencies when the base particle fractures orcrushes.

EXAMPLE 11

A protected particle was formed of a base particle having an averagediameter of 40 μm. The base particle is a hollow sphere having a shellthickness of 1 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The density of the base particle is 0.38g/cc. The outer surface of the base particle was coated with a PVA thatincluded additive of nano-carbonyl iron to adjust the density of theprotected particle to 1.9 g/cc. The coating was applied to the baseparticle by suspension deposition. The coating thickness was 60 μm. Theprotected particle had a crush strength of about 8000 psi. The protectedparticles were subjected to well conditions to fracture or crush thebase particles. The acoustic sound or emission created by the fracturedor crushed base particles had a traceable harmonic resonant frequencypeak at 1,500 Hz.

EXAMPLE 12

A protected particle was formed of a base particle having an averagediameter of 20 μm. The base particle is a hollow sphere having a shellthickness of 1 μm. The interior pressure in the hollow cavity of thebase particle is 1000 psi. The density of the base particle is 0.38g/cc. The outer surface of the base particle was coated with a PVA thatincluded additive of nano-carbonyl iron to adjust the density of theprotected particle to 2.9 g/cc. The coating was applied to the baseparticle by suspension deposition. The coating thickness was 60 μm. Theprotected particle had a crush strength of about 8000 psi. The protectedparticles were subjected to well conditions to fracture or crush thebase particles. The acoustic sound or emission created by the fracturedor crushed base particles had a traceable harmonic resonant frequencypeak at 1,200 Hz. As is evident from Examples 11 and 12, the protectedparticle can be tailored by using certain pressures in the hollow cavityof the base particle and sizes of the base particles to create a certainfrequency or range of frequencies when the base particle fractures orcrushes. When two or more different types of protected particles thathave been tailored to generate different frequencies or ranges offrequencies when the base particles of the two or more differentprotected particles are fractured or crushed, the recorded multiplefrequency profile signals at the sensor arrays can be used to providetwo or more separate maps of the fractures and formations in a well.Such information can be used to provide increased accuracy to themapping of formations.

EXAMPLE 13

A protected particle was formed of a base particle having an averagediameter of 200 μm. The base particle is a hollow sphere having a shellthickness of 48 μm. The base particle was formed of degradable sodiumsilicate. The interior pressure in the hollow cavity of the baseparticle is 14.7 psi. The density of the base particle is 2.06 g/cc. Theouter surface of the base particle was coated with a polylactic acid(PLA) by fluid bed spray coating. The coating thickness was 0.5 μm. Theprotected particle had a crush strength of over 12,000 psi. Theprotected particles were added to a proppant slurry and constitutedabout 0.5 wt. % of the proppant slurry. The proppant slurry with theprotected particles was pumped into the fracturing zones of a well. Theouter coating of PLA degraded by hydrolysis in the well and the rate ofdegradation only increased to an appreciable rate once the PLAencounters the higher temperatures within the deep well's fractures(around 60-100° C.), and thereafter degraded within 24 hours whichexposed the readily degradable sodium silicate hollow spheres to thewell pressure, thus being hydrostatically crushed.

EXAMPLE 14

A protected particle was mixed with a fracturing proppant such that theprotected particles constituted about 1 wt. % of the proppant slurry.The proppant slurry was then pumped into the fracturing zones in a well.The use of the protected particles increased the recordable signal fromhydraulic fracture formation to a signal-to-noise ratio of 1.05. Thesubterranean noise was approximately 30 dB and the base recordedfracture noise was about 32 dB. The protected particle, after itscontrolled delay for signaling due to the fracturing or crushing of thebase particles, increased the recorded fracture formation microseismicnoise to a signal-to-noise ratio of 2.0, with peak frequencies recordedup to 60 dB. Such increases in signal-to-noise ratio were used toimprove the accuracy in the mapping of the formations in the well.

The protected particles in Examples 1-14 can be used to map wellformations or other types of formations by the pumping, insertion orplacement of the protected particle in the formation and then monitoringthe created signal or emission from the fractured or crushed baseparticle after the protected particle is located in the desired locationin the formation. As can be appreciated, the protected particles inaccordance of the present invention can have other uses. For example,the protected particle can be used to monitor subterranean pressuresthrough the well and across different regions in the well as illustratedin Example 15.

EXAMPLE 15

A protected particle was formed of a base particle having an averagediameter of 500 μm. The base particle is a hollow sphere having a shellthickness of 10 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The outer surface of the base particle wascoated with a polyester by chemical vapor deposition. The coatingthickness was 120 μm. The protected particle had a crush strength ofabout 7000 psi. The protected particles were subjected to wellconditions to fracture or crush the base particles. The acoustic soundor emission created by the fractured or crushed base particles had atraceable harmonic resonant frequency peak at 550 Hz. The polyestercoating was used because of its slow degradation rate in well formation.For this application of the protected particles, the base particle inthe protected particles was to be fractured or crushed when theprotected particle encountered a pressure in the formation that wasgreater than the crush strength of the protected particle (e.g., thecrush strength of the protected particle prior to any significantdegradation of the outer coating). In use, when the protected particleis pumped into the subterranean fractures of the well, the protectedparticle will eventually encounter a well pressure that is greater thanthe crush strength of the protected particle, thereby resulting in thebase particle being fractured or crushed, thereby creating an acousticsignal or emission that can be detected. By using this technique,protected particles having known crush strengths can be used to map thepressures in a well formation by monitoring such certain protectedparticles creating an acoustic signal or emission that can be detected.As illustrated in Example 16, the frequency of the protected particlecan be tailored to distinguish protected particles having differentcrush strengths.

EXAMPLE 16

A protected particle was formed of a base particle having an averagediameter of 500 μm. The base particle is a hollow sphere having a shellthickness of 10 μm. The interior pressure in the hollow cavity of thebase particle is 14.7 psi. The outer surface of the base particle wascoated with a polyester by chemical vapor deposition. The coatingthickness was 100 μm. The protected particle had a crush strength ofabout 6000 psi. The protected particles were subjected to wellconditions to fracture or crush the base particles. The acoustic soundor emission created by the fractured or crushed base particles had atraceable harmonic resonant frequency peak at 500 Hz. The polyestercoating was used because of its slow degradation rate in well formation.For this application of the protected particles, the base particle inthe protected particles was to be fractured or crushed when theprotected particle encountered a pressure in the formation that wasgreater than the crush strength of the protected particle (e.g., thecrush strength of the protected particle prior to any significantdegradation of the outer coating). In use, when the protected particleis pumped into the subterranean fractures of the well, the protectedparticle will eventually encounter a well pressure that is greater thanthe crush strength of the protected particle, thereby resulting in thebase particle in the protected particle being fractured or crushed,thereby creating an acoustic signal or emission that can be detected. Byusing this technique, protected particles having known crush strengthscan be used to map the pressures in a well formation by monitoring suchcertain protected particles creating an acoustic signal or emission thatcan be detected. As can be appreciated, different types of protectedparticles can be used to map different pressures in a well formation.For example, the protected particles in Example 15 can be used to mapwhen the pressure in certain locations or regions of the well formationare about 7000 psi, and the protected particles in Example 16 can beused to map when the pressures at certain locations or regions in thewell formation are about 6000 psi. The different frequencies created bythese two protected particles when the base particle is fractured orcrushed can be used to identify which of the protected particles arefracturing or crushing in the well formation and the location in thewell of such protected particles.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained, andsince certain changes may be made in the constructions set forth withoutdeparting from the spirit and scope of the invention, it is intendedthat all matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense. The invention has been described with reference topreferred and alternate embodiments. Modifications and alterations willbecome apparent to those skilled in the art upon reading andunderstanding the detailed discussion of the invention provided herein.This invention is intended to include all such modifications andalterations insofar as they come within the scope of the presentinvention. It is also to be understood that the following claims areintended to cover all of the generic and specific features of theinvention herein described and all statements of the scope of theinvention, which, as a matter of language, might be said to fall therebetween. The invention has been described with reference to thepreferred embodiments. These and other modifications of the preferredembodiments as well as other embodiments of the invention will beobvious from the disclosure herein, whereby the foregoing descriptivematter is to be interpreted merely as illustrative of the invention andnot as a limitation. It is intended to include all such modificationsand alterations insofar as they come within the scope of the appendedclaims.

What is claimed is:
 1. A method of mapping subterranean formationscomprising: a. providing an acoustic imaging material, said acousticimaging agent material including a plurality of acoustic imagingparticles; each of said acoustic imaging particles includes a baseparticle mixed with a matrix material; each of said acoustic imagingparticles has a crush strength and is configured to be fractured orcrushed when pressure about said base particle exceeds said crushstrength of said base particle to produce a detectable acoustic signalor emission which can be used to provide information about asubterranean formation; said crash strength of said base particle is10-20,000% less than a crush strength of said acoustic imaging particle;said base particle is a hollow sphere; said hollow sphere includes oneor more materials selected from the group consisting of water glass,glass and ceramic; said hollow sphere has a diameter of 10 μm to 1 mm;said hollow sphere has a wall thickness of 0.1 μm to 200 mm; said hollowsphere has a crush strength of 100-19000 psi; said matrix material isformed of over 50 wt. % of one or more materials selected from the groupconsisting of polymer and metal; each of said base particle isconfigured to produce an acoustic signal or emission of about 0.4 Hz tobelow 10,000 Hz; a diameter of said acoustic imaging agent is up to 10mm; b. inserting said acoustic imaging material into said subterraneanformation; c. causing one or more of said acoustic imaging particles tobe fractured or crushed due to a pressure about each of said acousticimaging particles exceeding a crush strength of said acoustic imagingparticles as said acoustic imaging particles flowing through saidsubterranean formation; and whereby said fracturing or crushing of saidacoustic imaging particles in said subterranean formation results insaid detectable acoustic signal or emission; d. detecting saiddetectable acoustic signal or emission caused by said fracturing orcrushing of said acoustic imaging particles in said subterraneanformation; and, e. using said detected acoustic signal or emission toprovide information about said subterranean formation.
 2. The method asdefined in claim 1, further including a step of causing said outercoating to at least partially degrade or dissolve by exposing saidacoustic imaging particles to certain environmental conditions tothereby reduce said crush strength of said acoustic imaging particleswhile in said subterranean formation.
 3. The method as defined in claim1, further including a step of mixing said acoustic imaging materialwith a proppant to form a slurry mixture and then pumping said slurrymixture into said subterranean formation, said acoustic imaging agentconstituting 0.01-1.0 vol. % of said slurry mixture.
 4. The method asdefined in claim 2, further including a step of mixing said acousticimaging material with a proppant to form a slurry mixture and thenpumping said slurry mixture into said subterranean formation, saidacoustic imaging agent constituting 0.01-1.0 vol. % of said slurrymixture.
 5. The method as defined in claim 1, wherein said base particleconstitutes 10-40 wt. % of each of said acoustic imaging particles. 6.The method as defined in claim 4, wherein said base particle constitutes10-40 wt. % of each of said acoustic imaging particles.
 7. The method asdefined in claim 1, wherein said outer coating of said acoustic imagingparticles includes one or more additives selected from the groupconsisting of reinforcing agent to increase said crush strength of saidacoustic imaging particles, degrading agent to increase a degradationrate of said outer coating, and dense particle additive to increase adensity of said acoustic imaging particles, said one or more additivesconstitute 0.001-30 wt. % of said outer coating, said one or moreadditives having a size of about 10 nm-1 mm.
 8. The method as defined inclaim 7, wherein a) said dense particle additive has a density of atleast 1.7 g/cc and constitutes about 0.001-30 wt. % of said outercoating and is added in sufficient quantities to said outer coating toincrease a density of said acoustic imaging particles by 0.5-5 g/cc, b)said reinforcing agent constitutes about 0.001-10 wt. % of said outercoating, said reinforcing agent increasing said crush strength of saidacoustic imaging particles by 10-2000%, and c) said degrading agentconstitutes about 0.001-10 wt. % of said outer coating, said degradingagent reducing a time of degradation of said outer coating by at least5%.
 9. The method as defined in claim 1, wherein said base particle hasa diameter or average width of about 10Φ m to 100 mm.
 10. The method asdefined in claim 1, wherein said coating thickness of said outer coatingis 10 nm to 1,000Φ m.
 11. The method as defined in claim 1, wherein saiddetected acoustic signal or emission providing a) well fractureinformation including proppant placement in said subterranean formation,and/or b) well pressure information in said subterranean formation.